Deep Pectoral Myopathy in Chickens

Deep pectoral myopathy is a disease of the inner breast muscle, also called the supracoracoideus or pectoralis minor. You may also hear it called green muscle disease or Oregon disease. The problem happens when this muscle swells after heavy use, but the tough tissue around it does not stretch enough. That pressure can cut off blood flow, leading to muscle death.

In chickens, this is seen most often in heavy meat-type birds with large breast muscles. It may affect one side or both sides. Early on, the muscle can look pale, swollen, and bruised. Later, it often turns dry and greenish, which is where the common name comes from.

For many birds, there are few obvious signs while they are alive. The condition is often found only during processing or necropsy. In pet chickens or small flocks, some birds may show breast soreness, reduced comfort with wing movement, or a flattened area over the breast as the damaged muscle scars down.

This condition is usually a muscle injury syndrome, not an infectious disease. That matters because flock management and handling changes are often more useful than medications alone.

The main trigger is vigorous or prolonged wing activity. The deep pectoral muscle lifts the wing. When that muscle works hard, it swells. In chickens bred for heavy breast muscle, the surrounding fascia is tight and the muscle sits in a confined space between the breast and sternum. If swelling becomes severe, blood vessels are compressed and the muscle loses oxygen.

Common flock triggers include rough catching, struggling during restraint, sudden loud noise, predator scares, feed or water interruptions that make birds more active, and situations where a lame bird uses its wings to help move around. Birds placed on their backs may also flap hard enough to injure the muscle.

Risk is higher in heavy meat-type chickens, especially larger, faster-growing birds and higher-yield strains. Reports also note increased occurrence with heavier market weights, and some sources describe more cases in males. Free-range birds can also be affected if they have repeated bursts of wing activity.

This is not usually caused by bacteria, viruses, or parasites. Instead, it is best understood as an ischemic muscle injury related to anatomy, growth, and exertion.

Your vet starts with a history and physical exam. Helpful details include the bird's age and type, whether it is a heavy meat bird, and whether there was a recent fright, predator event, transport, catching session, or rough wing flapping. On exam, your vet may feel the breast for asymmetry, flattening, firmness, or pain.

A firm diagnosis in a live chicken can be difficult because the damaged muscle sits deep under the outer breast muscle. In many cases, the condition is suspected based on history and exam rather than confirmed immediately. Your vet may also look for other causes of pain, weakness, or breast swelling, including trauma, abscesses, fractures, or systemic illness.

If a bird dies or is humanely euthanized, necropsy is often the clearest way to confirm the diagnosis. The deep breast muscle may appear pale and swollen early, then dry, sharply outlined, and green later on. Histopathology may be used in some cases to confirm muscle necrosis and fibrosis.

Because many backyard chickens have overlapping signs from other conditions, diagnosis is really about ruling in a likely muscle injury while ruling out more urgent or contagious problems. That is why a veterinary exam still matters, even when treatment is mainly supportive.

Prevention focuses on reducing sudden, forceful wing flapping. Handle chickens calmly, keep wings folded against the body during restraint, and avoid placing birds on their backs when possible. Minimize loud disturbances near the coop, especially machinery, barking dogs, or predator pressure that can trigger panic.

Good flock management also matters. Keep feed and water consistently available, reduce crowding, and make movement through the environment predictable. If you raise heavy meat birds, be extra careful during catching, transport, and any event that causes struggling.

Watch lame or weak birds closely. Chickens that use their wings to help move around may be at higher risk for deep breast muscle injury. Early veterinary attention for mobility problems can help reduce secondary strain.

For backyard flocks, the goal is not to eliminate all activity. It is to lower the chance of sudden overexertion in a muscle that cannot safely expand. Small handling changes can make a meaningful difference.